1. Field of the Invention
The present invention relates to an error rate measurement apparatus for a mobile radio communications system, and more specifically, to an error rate measurement apparatus for measuring an error rate of received signals on a real-time basis in a mobile radio communications system.
The measured error rate is used for transmission power control, for example, which manages the output power of a transmitter so that the error rate be maintained at a constant level.
2. Description of the Related Art
In designing radio communications equipment, pursuing a perfect error-free transmission will normally end up with high costs of receiver and transmitter devices, intolerable power consumption, or inefficient channel utilization. Rather, it is more practical and efficient to design a system so as to maintain some permissible error rate for each type of information to be transferred. Error rates in the order of 10.sup.-3 are commonly acceptable in voice communications and 10.sup.-5 in data communications.
In mobile radio communications, the error rate of reception signals will exhibit intensive fluctuation due to wide variations of transmission path conditions. To maintain the above-described permissible error rate, the output power of transmitters should be controlled according to the error rate of reception signals.
FIG. 29 shows an example of error rate characteristics when transmission power is varied, in which the term "BER" stands for "bit error rate" and the reference power level (i.e., 0 dB) is based on BER=10.sup.-3. The scale of BER uses such expressions as "1e-05" implying "10.sup.-5." Assume here that a voice communications system has such characteristics as FIG. 29. When its error rate is found to be as high as 10.sup.-2, for example, the communications system will raise the transmission power by 8 dB to reduce the error rate.
In conventional radio communications systems, their error rate is measured through a test process in which some known data is transferred over a vacant channel. That test data is previously agreed upon by both the transmitter and receiver. The error rate is then obtained by examining the received signals. This method, however, fully occupies one channel for measurement in addition to the one for main communication purposes, and the channel utilization factor will thus deteriorate.
To solve this problem, another error rate measurement system is proposed in Japanese Patent Laid-open Publication No. 1-297924 (1989). In this proposal, a particular bit pattern for frame synchronization, which is inserted in each channel data signal, serves as a known test data for measurement.
To solve the same problem, still another measurement system is disclosed in Japanese Patent Laid-open Publication No. 3-222554 (1991), which system estimates the error rate by measuring the signal levels and phase shift of reception signals.
The former prior-art measurement system, however, has a problem with its measurement cycle time. To yield a reliable and confident result in terms of statistical analysis, one hundred or more faulty bit samples have to be collected. Since the frame synchronization pattern used in the former measurement system provides only a small population, the measurement should be done over many transmission frames. Consequently, it takes much time to obtain a reliable measurement result.
On the other hand, the latter prior-art measurement system has an accuracy problem by nature. In addition, it should handle analog values that will require much processing power to perform enormous and complicated computation.
Code division multiple access (CDMA) has been of major interest for future applications in mobile communications, since it provides enough strength against frequency-selective fading by using a wide-spread frequency range for signal transmission. In a radio communications system with this CDMA method, the error rate of reception signals will directly affect its channel capacity as shown in FIG. 30. Here, the term "channel capacity" denotes the number of communication channels available in each unit of coverage area, or a cell. More specifically, the FIG. 30 indicates that the bit error rate of 10.sup.-3 permits concurrent use of 500 channels in the same cell, while the bit error rate of 10.sup.-4 only allows 120 channels. As such, in the CDMA mobile communications, the bit error rate and channel capacity are so closely related to each other that a quick and accurate error rate measurement is seriously demanded.